Hello Mohain;
In theory this is easily done.

You can configure up to 31 slaves without needing extra hardware (such as repeaters) except for Profibus connectors and cable.

Just in case, since you do not want to use CPs: do all the S7-300s have an integrated DP port, and are they all without their own Profibus configuration (no slaves)? If not then you would probably need a CP 342-5 to set them up as Profibus slave nodes.

The easiest way to do it (if you do not need CP-342-5s) is to have a single project where all the nodes (S7-400 master and S7-300 slaves) are all included. The procedure is available here:

Inserting an I slave in a STEP 7 multiproject

See the following links for sample configurations of a DP netwoerk with a S7-400 as master and a S7-300 as slave:

DP link between CPU 315-2DP (slave) and S7-400 (master) with STEP 7 V5.3

What master/slave communication options are available on PROFIBUS DP?

But it is also possible to set up if the nodes are in seperate projects:

Master-slave connection between two CPUs in different projects

Hope this helps,
Daniel Chartier